Although 2–5 Hz and theta oscillations were most prominent in the PFC and HC, respectively, they were also observed in the two other structures, including the nRE, where they followed similar RPO stimulation-dependent variations in their frequency and power (Fig. 3). To test the possibility of the common origin of each, i.e. PFC for 2–5 Hz and HC for theta rhythm, we next calculated pairwise correlations between peak oscillatory power in different areas. The first analysis included all stimulation episodes (>450 in 15 rats; Fig. 4A–B) and showed significant (p < 0.001) relationship between each LFP pair, at both frequencies, correlations for 2–5 Hz being consistently stronger than for theta (Fig. 4C). The largest correlation was found between PFC and nRE (r = 0.82), about 30% larger than for the two other pairs (r = 0.5 for both PFC-HC and HC-nRE). Similarly, the strongest theta correlation was between nRE and the presumed origin of theta rhythm (p = 0.37 for HC-nRE vs. p = 0.2 for the other two LFP signal pairs) (Fig. 4C).
